spl: Convert spl_usb_load_image() to use linker list

Add a linker list declaration for this method and remove the explicit
switch() code.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/common/spl/spl_usb.c b/common/spl/spl_usb.c
index f990336..2bc321a 100644
--- a/common/spl/spl_usb.c
+++ b/common/spl/spl_usb.c
@@ -22,7 +22,7 @@
 static int usb_stor_curr_dev = -1; /* current device */
 #endif
 
-int spl_usb_load_image(struct spl_boot_device *bootdev)
+static int spl_usb_load_image(struct spl_boot_device *bootdev)
 {
 	int err;
 	struct blk_desc *stor_dev;
@@ -61,3 +61,4 @@
 
 	return 0;
 }
+SPL_LOAD_IMAGE_METHOD(0, BOOT_DEVICE_USB, spl_usb_load_image);